where each NAME=>TYPE pair declares one element of the struct. Each element
name is defined as an accessor method, unless a method
is explicitly defined with that name. (In that case, a warning is issued if the
-w flag is set.)

There are four possible element types, each represented by a string.
Each string may start with an asterisk (*), indicating that a reference
to the element is to be returned. The type of an
element determines the accessor method provided. The following list shows
the element types, the strings that represent them, and the accessor:

array (@or*@)

The element is an array, initialized to ().
With no argument, the accessor returns a reference to the element's
whole array.
With one or two arguments, the first argument is an index specifying
one element of the array; the second argument, if present, is the value
to be assigned to that array element.

class (Class_Nameor*Class_Name)

The element's value must be a reference blessed to the named class
or to one of its subclasses. The element is initialized to the
result of calling the new constructor of the named class.
The accessor's argument, if any, is the value to be
assigned to the element. The
accessor croaks if it's not an appropriate object reference.

hash (%or*%)

The element is a hash, initialized to ().
With no argument, the accessor returns a reference to the element's
whole hash.
With one or two arguments, the first argument is a key specifying
one element of the hash; the second argument, if present, is the value
to be assigned to that hash element.

scalar ($or*$)

The element is a scalar, initialized to undef.
The accessor's argument, if any, is assigned to the element.